1. Technical Field of the Invention
In general, the present invention relates to apparatus for processing a digital image from a medical diagnostic imaging device. More particularly, the present invention relates to apparatus for processing digital images, which may be of different bit lengths, by means of a window width/level look-up table and a curve shape look-up table, respectively, in the input and output data paths of a buffer image memory.
2. Background Art
In printing a hard copy from a digital image displayed on a video display device, it is desirable to process the digital image so that the hard copy image matches the appearance of the displayed image. Image processing is generally effected with a look-up table which is loaded with an appropriate image transfer function. An input pixel digital value is applied to the look-up table as an address and the output pixel digital value is supplied as the processed pixel digital value. Because the tone scale characteristics of a video display monitor are generally different from the tone scale characteristics of hard copy material (such as photosensitive film), a transfer characteristic which has a non-linear curve shape (such as illustrated in FIG. 1) is used. The following patents disclose the use of look-up tables to correct tone scale: U.S. Pat. No. 4,473,849, issued Sep. 25, 1984, inventor J. K. Cool; U.S. Pat. No. 4,794,460, issued Dec. 27, 1988, inventor K. Schiota; and U.S. Pat. No. 4,730,214, issued Mar. 8, 1988, inventors T. W. Lambert et al. The latter patent discloses a technique for developing a set of calibration data which can be stored in a look-up table to effect a match between an image displayed on a video monitor and a hard copy reproduction of that image on film. Generally, to prevent contouring of the transformed digital image, the output pixel value from the look-up table includes more bits than the input pixel digital value. For example, for an 8-bit input pixel, it is common to supply a 12-bit output pixel. Thus, an image memory used to buffer a digital image prior to printing must be able to store 12-bit pixels.
Modern medical diagnostic imaging includes the production of images, which are digitized, by various well-known imaging modalities such as ultrasound, magnetic resonance imaging (MRI), nuclear medicine, computed tomography (CT), digital subtraction angiography (DSA), and digital radiography. Such digital images are displayed on a video monitor. The technique of window processing was developed to improve the diagnosis of a region of interest in a diagnostic image. Because the tonal range of a region of interest may be small compared to the tonal range of the entire digital image, insufficient contrast in the region of interest may inhibit proper diagnosis. By expanding the tonal range in the region of interest to the entire tonal range of the display device through windowing, image contrast in the region of interest is greatly enhanced. Proper diagnosis is therefore greatly facilitated. The "window width" is the range of code values in the input digital image which is displayed over the full tonal range of the output display device. The "window level" defines the location of the window within the tonal range of the entire digital image. The window level may equal the minimum or center value of the window.
Typically, a digital image from a medical diagnostic imaging modality is processed for window width and level by means of a window width/level look-up table. Patents disclosing the use of window width/level processing in digital imaging devices include: U.S. Pat. No. 4,755,954, issued Jul. 5, 1988, inventor Z. Netter; U.S. Pat. No. 4,688,175, issued Aug. 18, 1987, inventors M. Kaneko et al.; U.S. Pat. No. 4,833,625, issued May 23, 1989, inventors H. D. Fisher et al.; U.S. Pat. No. 4,399,509, issued Aug. 16, 1983, inventor G. N. Hounsfield; and U.S. Pat. Nos. 4,680,628, issued Jul. 14, 1987, inventors D. R. Wojcik et al. and 4,730,212, issued Mar. 8, 1988, inventors D. R. Wojcik et al.
In a known laser printer for producing an image on film of an image from a medical diagnostic imaging modality, curve shape, window width and window level image processing have been combined into a single look-up table. This is illustrated in FIG. 3. A digital image signal from an image source 4 (such as a medical diagnostic imaging modality), is supplied to window width/level and curve shape look-up table (LUT) 6. The output from LUT 6 is stored in buffer image memory 8. Typically, the digital image from image source 4 may comprise an 8-bit digital signal while the output of LUT 6 is a 12-bit digital signal. Thus, a greater number of bits needs to be stored in memory, resulting in an increase in the size of the buffer image memory, and consequently, in an increase in the manufacturing cost. Moreover, combining the curve shape and window level image processing into a single look-up table is disadvantageous when several images are printed on a single page of film. Generally, curve shape processing is constant for all of the images on a page, whereas window width/level processing is specific to an image and therefore can change within a page. Thus, undesirable rapid changing of the window width/level image processing LUT would be required during printing of the film.
Because different medical diagnostic imaging modalities produce images having pixels of different bit lengths (such as 8-bit or 12-bit pixels), it is desirable that both image processing look-up tables and buffer image memory have the capability of handling, in the most efficient manner, digital images having pixels of different bit lengths. Typically, the image processing look-up table and buffer image memory are sized to handle a digital image having pixels with the greatest bit lengths to be processed. When digital images with fewer bit pixels are handled, this results in a waste of both look-up table space and image memory space with a resultant waste of manufacturing cost.